Thermal Compressor for On-orbit Refueling

The recent cost reduction of space cargo delivery has prioritized tasks that were previously deemed economically unfeasible. One such task is spacecraft refueling on orbit. The process relies on new technology: transfer of fluids under zero-gravity with constraints of space, such as minimal mass and power consumption, passivity, high reliability, stringent cleanliness standards. Spacecraft thermal systems are typically aimed at thermal stabilization. However, alternating heat fluxes produced by a dedicated thermal circuit can be a valuable driving force for applications where other forms of actuation become inferior. For instance, for pumping the difficult to handle fluids. High-purity noble gases are primary fuel for spacecraft ionic thrusters. Nowadays on-orbit refueling becomes an alternative to carrying single-use tanks charged on ground for whole mission. The systems must securely pump high-purity fluids against high-pressure gradients. Any mechanical system accomplishing such task faces unbearable challenges: contamination, leaks etc. Refueling system based on the gas thermal compression has no disadvantages associated with mechanical pumping. The thermal compressor was specifically developed for xenon refueling as part of the ESPRIT module for the "Gateway" station. Several engineering models of the compressor have been manufactured and successfully tested. The compressor functions as a high-performance heat exchanger between the thermal circuit and xenon. Designed to handle several tons of xenon at pressures of several hundred bars, it is currently undergoing a qualification campaign. A part of this campaign is the qualification of innovative manufacturing processes that enable the compressor to withstand plurality of pressure cycles, thermal shocks and thermal expansion/contraction cycles, while surviving launch phase vibrations. The compressor offers remarkable versatility. Its wide operational temperature (-85°C to 215°C) and pressure (0-200 bar) ranges, entirely metallic construction, absence of moving parts make it highly suitable for pumping a variety of fluids on orbit, including krypton.